The present invention relates to a data transmission device for motor vehicles, comprising a pulse generator and a monitoring unit, as well as a pulse generator for the monitoring unit.
Pulse generators are customarily introduced into a vehicle transmission and, by the detection as a function of time of the change between tooth and tooth-gap of a rotating gear wheel, generate an electric signal which is modulated in proportion to the speed of the vehicle. This signal is fed to a monitoring unit for recording and evaluation, the monitoring unit being developed in road traffic, for instance, as recording speedometer, taximeter, log or similar device.
Such pulse generators generally have, in addition to the sensor element, a circuit arrangement for the processing of a first signal and a signal driver in order to prepare the signal detected by the sensor element, which is initially still subject to numerous disturbing influences, for the feeding thereof by wire to the monitoring unit in the rough environment of a motor vehicle. The supplying of current to electronics of the pulse generator is generally effected in this connection from the monitoring unit over electric connecting lines.
In the event that the monitoring unit is developed as a recording speedometer, the signal detected by the pulse generator is evaluated for monitoring uniting compliance with the social-welfare regulations length of driving and periods of rest for drivers and/or compliance with traffic regulations concerning with respect to the maximum speed permissible for the vehicle. For this purpose, the signal detected by the pulse generator is, as is known, frequently subjected to various manipulations in order generally to exert an illegal influence on the path of the transmission of the signal from the pulse generator to the monitoring unit. The countermeasures which have been applied up to now, which are directed at simultaneously transmitting an inverted signal with respect to the detected signal over another parallel line provided in addition to the signal line and then evaluate the two signals comparatively in the monitoring unit, have proven insufficient, since devices have become known which, when introduced into the transmission path from pulse generator to monitoring unit are able to simulate this double pulse of said monitoring unit without the illegal manipulating being detectable by recording in the monitoring unit.